X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism investigation of size effects on field-induced N\'eel-cap reversal

X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on N\'eel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of N\'eel-cap magnetization give clear evidence for the magnetization reversal of the N\'eel caps around 120 mT, with a $\pm$20 mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots

Controlled switching of N\'eel caps in flux-closure magnetic dots

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called N\'eel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected.Comment: 4 pages, 3 figure

Artificial Kagome Arrays of Nanomagnets: A Frozen Dipolar Spin Ice

Magnetic frustration effects in artificial kagome arrays of nanomagnets are investigated using x-ray photoemission electron microscopy and Monte Carlo simulations. Spin configurations of demagnetized networks reveal unambiguous signatures of long range, dipolar interaction between the nanomagnets. As soon as the system enters the spin ice manifold, the kagome dipolar spin ice model captures the observed physics, while the short range kagome spin ice model fails.Comment: 4 pages, 4 figures, 1 tabl

Magnetostructure of MnAs on GaAs revisited

The ferromagnetic to nonferromagnetic (α-β) phase transition in epitaxial MnAs layers on GaAs(100) is studied by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism photoemission electron microscopy in order to elucidate the nature of the controversial nonferromagnetic state of β-MnAs. In the coexistence region of the two phases the β phase shows a clear XMLD signal characteristic of antiferromagnetism. The nature and the possible causes of the elusiveness of this magnetic state are discussed

How many bits may fit in a single magnetic dot? XMCD-PEEM evidences the switching of Néel caps inside Bloch domain walls

Elettra Highlights 2008-2009Data storage relies on the handling of two states, called bits. The market of mass storage is currently still dominated by magnetic technology, hard disk drives for the broad public and tapes for massive archiving. In these devices each bit is stored in the form of the direction of magnetization of nanosized magnetic domains, i.e. areas of ferromagnetic materials displaying a uniform magnetization. While miniaturization is the conventional way to fuel the continuous increase of device density, disruptive solutions are also sought. To these pertain in recent years many fundamental studies no longer considering the magnetic domains themselves, but the manipulation of the domain walls (DWs) that separate such domains. Concepts of storage and logic based on the propagation of DWs along lithographically-patterned stripes have been patented, while many fundamental aspects of DW propagation deeply related to condensed matter physics are still hotly debated. If one now considers magnetic dots of submicrometer dimensions, the magnetization has a tendency to curl along the outer edges of the nanostructure to close its magnetic flux and thereby reduce its magnetostatic energy. Then both domains and DWs of well-defined geometries arise, whose combined manipulation has been proposed as a multilevel magnetic storage scheme..

Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy

International audienceThe properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor

Caractérisation microbiologique et physicochimique du lait cru et de ses dérivés traditionnels Marocains (Lben et Jben)

Two hundred eighty-eight samples of traditional dairy products, including a third of raw milk, a third of « Lben » (traditionally fermented skimmed milk) and a third of « Jben » (traditional soft white cheese), were studied for their microbiological and physicochemical quality. These samples were collected from eight traditional dairies belonging to various sectors of Fez city, situated in the northern center of Morocco between October 2009 and September 2010. Microbiological and physicochemical analyses were performed according to Moroccan standards. Microbiological results of this study showed high and variables rates among the three dairy products for the majority of analyzed microorganisms, total aerobic mesophilic flora (FMAT), Total Coliforms (CT), Fecal Coliform (CF), Intestinal enterococci ( EI), yeasts (Lev), Moulds (Mois), Lactic Acid Bacteria (LAB), Staphylococcus aureus (S. aureus), sulfite-reducing anaerobes (ASR) and Salmonella (Salm). A seasonal effect was observed with an increase in microbial load of the majority of germs during the summer. According to microbiological criteria, set by Moroccan standards for dairy products, the overall rate of non-conformity for these three dairy products was 67%, with 89% of non-conformity in the samples of « Lben », 69% in samples of raw milk and 50% in samples of « Jben ». The average pH and titratable acidity of the three products were respectively 6.6 and 16 ° Dornic for raw milk, 4.5 and 80 ° Dornic for « Lben » and 4.3 and 95 ° Dornic for « Jben ». Generally, the three traditional dairy products marketed in the city of Fez have an unsatisfactory hygienic quality, probably due to the failure of following good hygiene practices during milking, collecting and transporting raw milk, or during its transformation to « Lben » and « Jben » in traditional dairies.Deux cent quatre-vingt-huit échantillons de produits laitiers traditionnels dont un tiers de lait cru de vache, un tiers de « Lben » (lait fermenté écrémé traditionnel) et un tiers de « Jben » (fromage frais traditionnel) ont été étudiés pour leur qualité microbiologique et physicochimique. Ces échantillons ont été collectés dans huit laiteries traditionnelles de la ville de Fès située au centre nord du Maroc, entre octobre 2009 et septembre 2010. Les analyses microbiologiques ainsi que la détermination du pH et de l’acidité ont été réalisées conformément à la réglementation marocaine. Les résultats montrent des concentrations élevées et variables entre ces trois produits laitiers pour la majorité des micro-organismes recherchés (flore mésophile aérobie totale, coliformes totaux, coliformes fécaux, entérocoques intestinaux, levures, moisissures, bactéries lactiques, Staphylococcus aureus, Anaérobies Sulfito-Réducteurs et Salmonella. Un effet saisonnier a été observé montrant une élévation de population pour la majorité des germes pendant la période estivale. Selon les critères microbiologiques fixés par la réglementation marocaine pour les produits laitiers, le taux global de non-conformité pour ces trois produis laitiers a été en moyenne de 67%, avec respectivement 89%, 69% et 50% de non-conformité dans les échantillons du « Lben », du lait cru et du « Jben ». Le pH et l’acidité titrable moyens de ces trois produits ont été respectivement de 6,6 et 16° Dornic pour le lait cru, 4,5 et 80° Dornic pour le « Lben » et 4,3 et 95° Dornic  pour le « Jben ». Globalement, ces trois produits laitiers commercialisés dans la ville de Fès présentent une qualité hygiénique non satisfaisante.  Le non respect des bonnes pratiques d’hygiène aussi bien lors de la traite, que de la collecte ou du transport du lait cru, ou encore lors de sa transformation en  « Lben » et en « Jben » dans les laiteries traditionnelles est probablement à l’origine de ce constat

Transmission XMCD-PEEM imaging of an engineered vertical FEBID cobalt nanowire with a domain wall

Using focused electron-beam-induced deposition, we fabricate a vertical, platinum-coated cobalt nanowire with a controlled three-dimensional structure. The latter is engineered to feature bends along the height: these are used as pinning sites for domain walls, which are obtained at remanence after saturation of the nanostructure in a horizontally applied magnetic field. The presence of domain walls is investigated using x-ray magnetic circular dichroism (XMCD) coupled to photoemission electron microscopy (PEEM). The vertical geometry of our sample combined with the low incidence of the x-ray beam produce an extended wire shadow which we use to recover the wire''s magnetic configuration. In this transmission configuration, the whole sample volume is probed, thus circumventing the limitation of PEEM to surfaces. This article reports on the first study of magnetic nanostructures standing perpendicular to the substrate with XMCD-PEEM. The use of this technique in shadow mode enabled us to confirm the presence of a domain wall without direct imaging of the nanowire